There are two different questions being asked here (and a third not being addressed)
1 - How do you make sure you do not accidentally run a tank dry on a longish flight in an aircraft that feeds from only one tank at a time.
- Lots of good answers in the thread
2 - How do you Actually know how much fuel is left at any time
- Fewer good answers (IO's definitely, FougaMagister seems to fly the same or similar aircraft multiple times so uses a close approximation of IO's)
On many aircraft, fuel burn is highly dependant on leaning technique which is one of the reasons pilots are often taught not to trust the book numbers. FMS commercial technique applied by a less experienced pilot without accurate leaning could easily wind up as a glider on a flight that actually had to execute the diversion
3 - (The one not asked yet) How do you operate a maximum range flight. Ie. How much usable fuel is really in your tanks (more important for bladder than integral tanks) and what fuel configuration do you want on landing.
Deakin has an article on Pelican's perch (AvWeb) where he advocates running each tank dry (on separate flights!) so on refilling he can establish exactly how much useable fuel there was, He then advocates, if you have to stretch every drop out (i.e. an emergency) running one of the tanks dry so you have all of your fuel in one tank on landing and minimise the risk of un-porting or being on the slightly less full tank and accidentally running it dry in the pattern. Finally of course as usual, he recommends a full engine analysis and a known calibrated fuel totalizer. And finally, there is an implied caution to make sure you have plenty of enroute diversion options